The question is whether the brain is the be-all and end-all of the matter of personal identity for human persons6.

It is acknowledged by most that – conceptually at least – there can be persons7 that are not humans (ie. not members of the species homo sapiens) – whether these persons be non-human animals, computers, God, angels, aliens or whatever. Non-animals presumably have no brains, though aliens presumably have a brain-analogue, so brains cannot be identity-criteria for personhood as such (indeed, we might argue that there are no criteria for persons as such8). But for animal-persons (human or otherwise), the brain seems to occupy a central place, both as the seat of psychology (in the absence of an immaterial soul9) and as the regulator of the body.

So, the story would go, X is the same person as Y iff10 X has the same brain as Y.

The trouble is – even if this claim is along the right lines – we can press matters further, and ask whether the whole brain is strictly necessary. If what impresses us is a brain-based psychological view, when what we imagine is “really the minimal me” is the pair of psychology-bearing cerebral hemispheres, then we might imagine (as some philosophers have) a case of fission11, where – after equalising the hemispheres in psychological potency, we transplant12 one into another body lacking both hemispheres. Or, without needing anything so radical, we sever the corpus callosum in a commissurotomy13, thereby (on this view) creating two persons in one body.

However, if we are animalists14,15 wondering what the “minimal animal” is, and it’s the command-and-control functions of the brain that impress us, then the paring-down process might16 be able to do without the cerebral hemispheres (or at least the psychology-bearing parts) altogether. So, brain-based views from different perspectives might come to different conclusions about the importance of the cerebral hemispheres – one view might make them essential, the other irrelevant to questions of identity (if not to “what matters17”). It is an empirical question whether the brain-stem can be divided, and hence whether a brain-based animalist approach is also subject to worries18 about fission.

Anyway, the appropriateness of the Brain criterion of personal identity depends on what we are19– in particular whether we are (most fundamentally, or in the sense of numerical identity, which is not the same thing) human animals or persons constituted by20 them (or various other things).

Only if21 we believe that we are (identical to) brains22 will we adopt the brain criterion.

Much of this discussion has empirical aspects to it, and depends on the capabilities of real brains – though we might get into the choppy waters of more intricate TEs, and wonder what might be the case if the biology went differently – but then we would most likely not be talking about our identity criteria, but of some other being.